Maraging stainless steel particularly for use in cast condition

ABSTRACT

A maraging stainless steel containing correlated amounts of nickel, chromium, silicon, and metal from the group consisting of aluminum and titanium, the steel being particularly suitable in cast form since it affords a combination of good strength, toughness and corrosion resistance and also exhibits excellent foundry characteristics.

United States Patent Floreen Oct. 23, 1973 MARAGING STAINLESS STEEL2,505,763 5 1970 GOllfll 75 124 PARTICULARLY FOR USE IN CAST 2,738,2675/1950 Pakkala 75/124 2,820,708 1/1958 Waxweiler.... 75/128 CONDITION3,278,298 10/1966 Perry 75/128 [75] Inventor: Stephen Floreen, Suffern,N.Y. 3,251,683 5/1966 Hammond t 75/l28 T 3,532,491 10/1970 Floreen75/128 T 1 Asslgneel The Intematloflal Nlckel Company, 2,801,916 8/1967Harris 75/128 T Inc., New York, N.Y.

[22] Filed: Sept 1971 Primary ExaminerHyland Bizot 2 Appl, 1 1,234Attorney-Maurice L. Pinel Related US. Application Data [63]Continuation-in-part of Ser. No. 865,969, Oct. 13,

1969, abandoned. [57] ABSTRACT A maraging stainless steel containingcorrelated [52] 75/128 75/124 T amounts of nickel, chromium, silicon,and metal from [51] Int. Cl. C22c 3 /20 the group consisting of aluminumand titanium, the [58] Field of Search 75/128 T, 128 N, Steel beingparticularly Suitable in cast form since it 75/124 affords a combinationof good strength, toughness and corrosion resistance and also exhibitsexcellent [56] References cued foundry characteristics.

UNITED STATES PATENTS 10/1964 Lula 75/124 17 Claims, No DrawingsMARAGING STAINLESS STEEL PARTICULARLY FOR USE IN CAST CONDITION Thisapplication is a continuation-in-part of application Ser. No. 865,969,filed Oct. 13, 1969.

Nearly a decade has passed since the original discovery of theso-designated maraging steels. And while research continues on anextensive basis there is not yet on the commercial scene, at leastinsofar as I am aware, a low cost, cast stainless maraging steel.Conventional cast versions, however, have long been in commercialproduction, for example, the cast maraging steel described in US. Pat.No. 3,132,937. Indeed, significant processing improvements have beenattained in respect of such cast steels as evident from the heattreating procedures set forth in US. Pat. No. 3,341,372. Nonetheless,such steels are, at least comparatively speaking, relatively costly and,more importantly, do not exhibit stainless characteristics.

Thus, there exists a hiatus in the maraging family of steels, to wit: acast stainless maraging steel which in one package offers low cost, highcorrosion resistance, good foundry characteristics and a tensilestrength on the order of, say, about 150,000 psi or more, coupled withgood toughness, although such steels with strengths as low as about125,000 psi are also quite useful for many applications.

It has now been discovered that the foregoing objectives can be attainedprovided the steels contain properly correlated amounts of nickel,chromium, silicon, metal from the group consisting of aluminum andtitanium, and other elements as set forth herein.

Generally speaking, where tensile strengths on the order of about150,000 psi and higher are necessary together with good corrosionresistance and toughness, the present invention contemplates caststainless maraging steels containing (in weight per cent) from percentto about 12.5 percent, e.g., 10 to 12 percent, chromium, about 7.5 to 11percent, e.g., 8 to 10 percent, nickel, the sum of the chromium plusnickel being at least 18 percent, e.g., 19 percent, but less than 22percent and advantageously not over 21.5 percent, about 1.5 to 3.percent silicon, a small but effective amount, e.g., 0.01 percent, ofmetal from the group consisting of aluminum and titanium, up to about 1percent manganese, up to about 0.05 percent carbon and the balanceessentially iron. For tensile strengths down to about 125,000 psi, lowerpercentages of silicon and nickel can be utilized as explained herein.Further, the sum of chromium plus nickel can be as low as 17.5 percent.Elements such as phosphorus, sulfur, oxygen and nitrogen should be keptat low levels consistent with good commercial steelmaking practice.

In carrying the invention into practice should the chromium content fallbelow about 10 percent corrosion resistance is impaired. Moreover,mechanical properties can be decidedly unattractive. On the other hand,at chromium levelsmuch above 12.5 percent, problems due to delta ferritecan arise, thus adversely affecting toughness characteristics. This isparticularly true with low percentages of nickel. For this reason, amongothers, in seeking the highest combination of strength and toughness,the nickel content should be at least 7.5 percent and is mostbeneficially at least 8 percent or 8.5 percent, since it both inhibitsdelta ferrite formation and markedly enhances toughness. However, inrespect of the lower strength steels, good results can be achieved atreduced percentages of nickel,

i.e., about 6 percent or 6.5 percent, particularly with silicon contentsbelow 1.5 percent. Silicon, as does chromium, also tends to promotedelta ferrite and as the amount thereof is reduced less nickel is thusrequired, other factors being equal.

Although nickel counteracts delta ferrite and promotes toughness andthough chromium imparts the primary resistant effect to variouscorrosive media, it has been found that together these constituents canbe present to such an extent that mechanical properties are subverted byreason of the fact that the alloys are undesirably characterized by lowM, temperatures. Therefore, the sum of the chromium plus nickel shouldbe less than 22 percent and in striving for optimum results should notexceed 21 percent or 21.5 percent. It might be pointed out that the M,temperature should not be lower than about 325F. to 350F. to thereby assure obtaining steels which upon transformation are characterized by anessentially martensitic structure upon cooling from say, hot working orannealing temperatures and the like.

With regard to silicon, it should not fall below about 1.5 percent wherethe emphasis is on the higher strength steels. Percentages much above 3percent silicon, while imparting strength, detract from toughness,particularly the ability of the steels to absorb impact energy. Asilicon range of 1.6 to 2.5 percent, e.g., 1.8 to 2.3 percent, is mostsatisfactory. Lower amounts of silicon can be used in connection withsteels having tensile strengths between about 125,000 and 150,000 psi;however, at least about 0.5 percent silicon, e.g., at least 0.6 percent,is necessary for good foundry characteristics and to facilitate ease ofcastability.

In the absence of either or both aluminum and titanium mechanicalproperties, particularly in respect to air melted steels, aredeleteriously affected. These constituents are deemed to tie upinterstitials (carbon, nitrogen) and such elements as oxygen, thepresence of which might otherwise bring about an undesirable loss intoughness. Only a small amount, e.g., 0.01 percent or 0.02 percent, ofeither or both of these constituents need be present and it is notnecessary that either exceed 0.1 percent or 0.2 percent or that thetotal amount exceed about 0.3 percent.- Preferably, the steels containnot more than about 0.2 percent of aluminum and/or titanium. In aimingfor optimum properties, from 0.02 to 0.07 percent .of each of theseelements has been found to afford excellent results.

While it is above indicated that manganese can be present in the steelsin accordance herewith in percentages up to about 1 percent, it isnonetheless extremelyadvantageous to maintain this constituent at muchlower levels, to wit: not above about 0.4 to 0.5 percent. As will bedemonstrated herein, manganese can impair the capability of the steelsto absorb high levels of impact energy. As to carbon, if toughnesscharacteristics are not to be needlessly sacrificed this element shouldnot be present in amounts above 0.05 percent and, indeed, should bemaintained at significantly lower levels, e.g., below about 0.03 percentor 0.02 percent.

For the purpose of giving those skilled in the art a betterunderstanding of the invention the following illustrative data aregiven.

A series of 30 pound air-induction melts utilizing electrolytic grademetals as starting materials was prepared. The furnace was first chargedwith iron and nickel together with about 0.05 percent carbon (carbonboil) for the deoxidation. Thereafter, about 0.1 percent each ofaluminum and titanium was added followed by the silicon addition. Theheats were cast in dry said double keel block molds, the leg of eachkeel being 1 inch X Hinches X 7 inches in length. Standard tensile 1inch) and Charpy V-Notch impact specimens were machined from the keelblock and were thereafter solution annealed about one hour at 1900F.,air cooled and maraged at about 850F. for about 3 hours. The results ofthese tests are reported in Tables I and II.

TABLE I Percent Ni C-r Si Al Ti Fe 6 .0 6 .2 1.86 .019 0 .04 0 .07 Bal.8 .0 8 .2 1.82 0 .008 0 .04 0 .07 Bel. 9.9 10.1 1.77 0.011 0.01 0.03Bal. 9.0 11.3 1 .88 0.014 0.02 0.02 Bal. 8 .0 10 .1 1.80 0 .021 0 .040.07 Bal. 8 .8 11 .9 1.84 0 .012 0 .02 0.03 Bal. 8 .1 12 .1 1.75 0 .0130 .02 0.03 Ba]. 7 .9 12 .0 0.92 0 .014 0 .02 0 .02 Bal. 8.0 12.1 2.320.008 0.03 0.06 Ba]. 7.9 12.2 2.77 0.025 0.02 0.04 Bal.

TABLE II Y.S.,* U.T.S E1ong., R.A., CVN, Alloy k.s 1 k.s 1 percentpercent ft.-lbs.

"=0.2% offset.

Concerning the above data, the effect of either low nickel and/orchromium will be observed from a pcrusal of Alloys A and B. Therelatively low impact strength of these alloys is in marked contrast tothose in accordance with the invention as manifested by Alloys 1 through8. Alloys 1 and 2 offered a particularly good combination of strengthplus toughness. Thus, alloys containing from about 10 to 12 percentchromium, 8.5 to 10 percent nickel and about 1.8 to 2.3 percent siliconare exceptionally good.

Tests were also conducted in which Armco iron, ferrochromium andferrosilicon were employed rather than the ultra pure electrolyticgrades of iron, chromium and silicon used in connection with Table I.

These tests were performed in much the same manner as described inconnection with the alloys of Table I, the compositions (Alloys 9 and 10are within and C and D are without the invention) and results beinggiven in Tables III and IV, respectively.

The data set forth in Tables III and IV indicate that the use ofexceptionally pure materials is not necessary. Alloy C confirms thatsignificantly lower levels of nickel (5 percent) subvert resistance toimpact. Alloy 10 reflects that as the nickel content is increased above9 percent in these less pure materials, impact resistance is notbenefitted. The low strength of Alloy D is deemed attributable toretained austenite. From overall considerations it is preferred tomaintain the nickel content at not more than 9 percent regardless ofsilicon content.

A number of alloys given in Table l were also exposed to corrosiontests. In this regard, panels of the maraged steels (about 1 inch X 4inches X %inch thick) were exposed to the well-known Salt Spray (Fog)Test in accordance with ASTM designation B1 17-61. Upon examination ofthe panels, they were re-tested in the more severe Copper-AcceleratedAcetic Acid-salt Spray (CASS) test (ASTM designation B368-61T). Sinceboth of these tests are generally well-known by those skilled in theart, a detailed description is omitted but is described inSpecifications and Tests for Electro-deposited Metallic Coatings. ASTMPhiladelphia 1961, 3rd ed. In any case, the results of the corrosiontests indicated that the low chromium Alloys A and B were much inferior,with variation in the silicon content seemingly having little effect.

Foundry characteristics were evaluated by pouring dry sand mold fluidityspirals at three different pouring temperatures, 2950F., 2850F. and2775F. The steel used nominally contained 11.5 percent chromium, 8.5percent nickel and 2 percent silicon, the balance being essentiallyiron. The resulting spiral lengths were 35 inches, 31 inches and 17inches,-respectively. Each of the spirals exhibited good mold fillingcapability. The freezing temperature of the steel was approximately2600F. Accordingly, in view of the pouring temperature and degree ofsuper-heat, the fluidity measurements indicated that the castability ofthe subject steels would be at least as good as if perhaps not betterthan standard cast CF-8 stainless steel.

As indicated above herein manganese exerts a detrimental influence withregard to steels of the present invention. This is illustrated by theimpact data given in .the following Table V.

TABLE V Percent CVN, Alloy Ni Cr Si 0 Al Ti Mn ft. lbs.

The cast steels of the present invention can be utilized for suchapplications as wearing rings, compressor wheels, corrosion resistantgears, high pressure valves, propellers, components for power plantpumps, including impellers, stage pieces, diffusers, etc., and forapplications generally requiring steels which manifest a goodcombination of corrosion resistance, strength and toughness.

While the steels above described have been set forth solely inconnection with applications as cast steels they also are useful in thewrought form. In this connection, if used in the wrought condition, thenickel content can be lowered to 5.5 percent or even 5 percent and thechromium content can be extended up to 15 percent, the sum of thechromium and nickel being less than 22 percent. This obtains over thefull silicon range.

It is to be understood that the expressions balance or balanceessentially used in referring to the iron content of the steels inaccordance herewith, are not intended to exclude the presence of otherelements, e.g., deoxidizing and cleansing constituents, and impuritiesnormally associated therewith, in small amounts which do not adverselyaffect the basic characteristics of the subject steels.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. A maraging stainless steel in the cast and martensitic conditionconsisting essentially of from about percent to about 12.5 percentchromium, about 7.5 percent to about 11 percent nickel, the sum of thechromium plus nickel being at least about 18 percent but less than 22percent, about 1.5 percent to about 3 percent silicon, metal from thegroup consisting of titanium and aluminum in a small but effectiveamount sufficient to enhance the toughness of the steel, the titaniumand aluminum not exceeding about 0.2 percent each, up to about 1 percentmanganese, up to 0.05 percent carbon and the balance iron.

2. An alloy in accordance with claim 1 containing from about 11 percentto about 12 percent chromium.

3. An alloy in accordance with claim 1 containing from about 8 percentto about 10 percent nickel.

4. An alloy in accordance with claim 1 in which the sum of the nickelplus chromium does not exceed 21.5

to 2.5 percent silicon.

6. An alloy in accordance with claim 1 in which aluminum is present inan amount from about 0.01 percent to about 0.1 percent.

7. An alloy in accordance with claim 1 in which titanium is present inan amount of from about 0.01 to 0.1 percent.

8. An alloy in accordance with claim 1 containing about 0.02 percent toabout 0.07 percent each of titanium and aluminum.

9. An alloy in accordance with claim 1 containing about 1 1 percent toabout 12 percent chromium, about 8.5 to 9.5 percent nickel, the chromiumplus nickel being from about 20 to 21.5 percent, about 1.8 percent toabout 2.3 percent silicon, up to about 0.5 percent manganese, up to 0.03percent carbon, and 0.01 to 0.1 percent of titanium and aluminum.

10. An alloy in accordance with claim 9 containing 0.02 to 0.07 percentof both titanium and aluminum.

11. An alloy in accordance wth claim 4 in which the chromium plus nickelis at least 19 percent.

12. An alloy in accordance with claim 11 containing up to 0.03 percentcarbon.

13. A maraging steel in the martensitic condition and consistingessentially of about 10 percent to about 15 percent chromium, from 5 to11 percent nickel, the sum of the chromium plus nickel being at least 18percent but less than 22 percent, about 0.5 percent to about 3 percentsilicon, metal from the groupconsisting of titanium and aluminum in asmall but effective amount sufficient to enhance the toughness of thesteel, the titanium and aluminum not exceeding about 0.2 percent each,up to about 1 percent manganese, up to 0.5 percent carbon and thebalance iron.

14. An alloy in accordance with claim 13 containing 11 to 15 percentchromium, 7.5 to 9.5 percent nickel, 1.8 to 2.3 percent silicon, metalfrom the group consisting of aluminum up to 0.1 percent and titanium upto 0.1 percent, up to 0.5 percent manganese, and up to 0.03 percentcarbon.

15. A cast maraging steel in the martensitic condition and consistingessentially of about 10 percent to about 15 percent chromium, from 5 tol 1 percent nickel, the sum of the chromium plus nickel being at least18 percent but less than 22 percent, about 0.5 percent to about 3percent silicon, metal from the group consisting of titanium andaluminum in a small but effective amount sufficient to enhance thetoughness of the steel, the titanium and aluminum not exceeding about0.2 percent each, up to about 1 percent manganese, up to 0.5 percentcarbon, and the balance iron.

16. A cast maraging steel in accordance with claim 15 containing from6.5 percent to about 8 percent nickel and from 0.6 to 1 percent silicon.

17. A cast maraging steel in accordance with claim 16 containing 0.01 to0.1 percent aluminum and 0.01

to 0.1 percent titanium.

2. An alloy in accordance with claim 1 containing from about 11 percentto about 12 percent chromium.
 3. An alloy in accordance with claim 1containing from about 8 percent to about 10 percent nickel.
 4. An alloyin accordance with claim 1 in which the sum of the nickel plus chromiumdoes not exceed 21.5 percent.
 5. An alloy in accordance with claim 1containing 1.6 to 2.5 percent silicon.
 6. An alloy in accordance withclaim 1 in which aluminum is present in an amount from about 0.01percent to about 0.1 percent.
 7. An alloy in accordance with claim 1 inwhich titanium is present in an amount of from about 0.01 to 0.1percent.
 8. An alloy in accordance with claim 1 containing about 0.02percent to about 0.07 percent each of titanium and aluminum.
 9. An alloyin accordance with claim 1 containing about 11 percent to about 12percent chromium, about 8.5 to 9.5 percent nickel, the chromium plusnickel being from about 20 to 21.5 percent, about 1.8 percent to about2.3 percent silicon, up to about 0.5 percent manganese, up to 0.03percent carbon, and 0.01 to 0.1 percent of titanium and aluminum.
 10. Analloy in accordance with claim 9 containing 0.02 to 0.07 percent of bothtitanium and aluminum.
 11. An alloy in accordance wth claim 4 in whichthe chromium plus nickel is at least 19 percent.
 12. An alloy inaccordance with claim 11 containing up to 0.03 percent carbon.
 13. Amaraging steel in the martensitic condition and consisting essentiallyof about 10 percent to about 15 percent chromium, from 5 to 11 percentnickel, the sum of the chromium plus nickel being at least 18 percentbut less than 22 percent, about 0.5 percent to about 3 percent silicon,metal from the group consisting of titanium and aluminum in a small buteffective amount sufficient to enhance the toughness of the steel, thetitanium and aluminum not exceeding about 0.2 percent each, up to about1 percent manganese, up to 0.5 percent carbon and the balance iron. 14.An alloy in accordance with claim 13 containing 11 to 15 percentchromium, 7.5 to 9.5 percent nickel, 1.8 to 2.3 percent silicon, metalfrom the group consisting of aluminum up to 0.1 percent and titanium upto 0.1 percent, up to 0.5 percent manganese, and up to 0.03 percentcarbon.
 15. A cast maraging steel in the martensitic condition andconsisting essentially of about 10 percent to about 15 percent chromium,from 5 to 11 percent nickel, the sum of the chromium plus nickel beingat least 18 percent but less than 22 percent, about 0.5 percent to about3 percent silicon, metal from the group consisting of titanium andaluminum in a small but effective amount sufficient to enhance thetoughness of the steel, the titanium and aluminum not exceeding about0.2 percent each, up to about 1 percent manganese, up to 0.5 percentcarbon, and the balance iron.
 16. A cast maraging steel in accordancewith claim 15 containing from 6.5 percent to about 8 percent nickel andfrom 0.6 to 1 percent silicon.
 17. A cast maraging steel in accordancewith claim 16 containing 0.01 to 0.1 percent aluminum and 0.01 to 0.1percent titanium.